Various prior fabrics and braided materials have been used in the manufacture of composite articles. For example, two-dimensional fabrics, whether braided, woven, or made by non-woven processes, are typically deployed in the manufacture of a composite part in multiple layers of material to build up predetermined thicknesses of material that may vary throughout the composite part. Prior conventional three-dimensional fabrics have been similarly used in the manufacture of composite parts. With two-dimensional fabrics without further processing, there are no tows that convey in-thickness loads from one layer of material to the next, i.e., there is no means of transmitting load transverse to the layers of fabric material except through the resin encasing the fabric, which by itself typically has limited ability to support the loading. Some measure of intertwining between the layers can be imparted into the structure by stitching or sewing additional materials through the layers. This intermediate or post-processing type of operation results in a pseudo three-dimensional structure providing some measure of cross-thickness load transfer; however, the known intermediate or post processing operations provide limited structure between the layers, and includes materials that are distinct from the in-thickness materials. The resulting load transfer typically remains through the resin encasing the fabric materials.
In this disclosure we use the term “tow” as a cluster or grouping of materials that extend together in a principal direction as a unit. Tows may be one fiber or a plurality of fibers. Tows may include monofilaments, multiple filaments or combinations of monofilament and multiple filament strands, and may be staple or spun materials. Tow materials can have a variety of cross-sectional shapes, including but not limited to, generally circular, ellipsoidal, triangular and flat tape shapes. Fibers forming a tow may be twisted, twined, braided or otherwise shaped or combined, or may extend contiguously without being twisted or twined together. Fibers forming tows may be coated with resin or other coating to facilitate braiding and/or subsequent processing. A tow can include any combination of materials and material forms. As examples, a tow may include all carbon materials, a combination of carbon and thermoplastic materials, or a combination of aramid and glass materials. Other combinations of tow materials are known and used in composite structures and may be used in the present invention.
Prior three-dimensional structures have tows providing cross-thickness load paths, which is in the radial direction in a tubular sleeve. Three prior methods of forming three-dimensional braids include (1) the 4-step process, (2) the two-step process, and (3) the multilayer interlock braiding process. The 4-step process is also known by other names such as row-and-column braiding, Omniweave, Magnaweave, and through-the-thickness braiding. The 4-step braiding machine has a flat or cylindrical bed moving tow carriers from predetermined point-to-point locations on a grid of rows and columns. In a first step, a group of tow carriers is moved within columns in directions that alternate column to column, a second step includes moving another group of tow carriers within rows in directions that alternate row to row. In third and fourth steps, these operations are carried out in reverse with or without involving the same groups of tow carriers. The four steps are repeated to form a braid, and the groups of tow carriers may change from one repetition to another. In various alternatives, additional tow carriers are added around the outside perimeter of the shape formed by the moving carriers. A mechanism is typically required in 4-step braiding to compact the tows into the braided form during the process to consolidate the braided structure as it is being formed. The 4-step process is exemplified by U.S. Pat. No. 4,312,261 Florentine.
The two-step three-dimensional braiding process includes a relatively large number of fixed tow carriers that deliver tows into an axial direction of the braided structure and a fewer number of moving tow carriers as compared to 4-step braiding. The two steps include first moving some group of tow carriers in alternate directions column to column, and second, moving another group tow carriers in alternate directions row to row. Unlike 4-step braiding, no mechanical means of compacting the tows into the braided form is typically required because the yarn tension serves this purpose. The two-step process is exemplified by U.S. Pat. No. 4,719,837 McConnell et al.
The multilayer interlocking three-dimensional braiding process uses a braiding machine that moves tow carriers in a way similar in configuration to a circular braiding machine used to manufacture conventional two-dimensional braids. However, in the multilayer interlocking process, rows of tow carrier conveyance devices, the most common being what are referred to as “horn gears”, are arranged in a Cartesian grid or in concentric circular paths around the longitudinal axis of the braiding machine. Then, the tow carriers move from one row to an adjacent row in a predetermined pattern. The multilayer interlocking process is exemplified by U.S. Pat. No. 5,388,498 Dent et al and U.S. Pat. No. 5,501,133 Brookstein et al.
Prior multilayer interlocked braids tend to provide intertwined tows primarily in the plane of the braid structure similar to the way tows are in a conventional two-dimensional braid structure. This typically results in better in-plane properties of the braided structure than 4-step and two-step braids, but less radial or cross-thickness strength. The 4-step and two-step braids typically allow for a greater density of tows in the braided structure and produce a greater degree of intertwining in the radial or cross-thickness principal directions, but typically provide less in-plane strength.